Recovery of zinc



Patented July I 1929.

UNITED STATES PIOXTENT OFFICE.

WILLIAM GRENVILLE HORSCH, OF CORAOPOLIS, PENNSYLVANIA, ASSIGNOR TO VUL- CAN DETINNING COMPANY, OF SEW-AIKEN, NEW JERSEY, A CORPORATION OF NEW I JERSEY.

RECOVERY OF ZINC.

' No Drawing.

iron, and has for its object'the provision of Application filed December 2, 1926. Serial No. 152,305.

dissolve a perfectly definite amount of zinc oxide, resulting in a certain definite zinc concentration. Such a solution, which has an improved method of recovering'zinc from dissolved all of the zinc oxide that it will such materials. More particularly, the object of the invention is to provide an improved method of recovering zinc from galvanized iron, zinc dross and similar zinc and iron bearing materials.

to The invention contemplates a substantially complete separation of the zinc from the iron in zinc and iron bearing materials, so that the iron remains in condition to be readily utilized for remelting. The zinc is preferably recovered in the form of an intermediate zinc oxide product, of a high degree of purity, which is readily converted into commercial products such as zinc chloride,

zinc sulfate, etc.

Tn accordance with the invention, the zinc and iron bearing material is subjected to the action of an appropriate solvent that dissolves zinc but not iron and which enters the metastable condition as the zinc concentration increases. A solution of an alkali metal zincate containing an excess of alkali is a suitable solvent for this purpose. The presence of an oxidizing agent, such as an alkali metal nitrate, promotes the solution W of the zinc. The resultin pregnant solution is appropriately treated or the recovery of a zinc-bearing product therefrom.

in the preferred practice of the invention, the separation of the zinc from the iron is brought-about by means of an alkaline sodium zincate solution of the proper absolute and relative concentrations of'zinc and free sodium hydroxide. Thus, the concentration of total sodium hydroxide may lie within the range of 200 to 400 grams per liter, while the zinc concentration, which is relatively low at the start, rises as solution or stripping progresses to from 100 to 200 grams per liter. An excess of sodium hydroxide should be present in this solution; in other words, the solution is always alkaline. The recovery of the zinc in the form of a zinc oxide product from the resulting charged or pregnant solution by appropriate means leaves the alkaline sodium zincate solution in a regenerated condition for the treatment of fresh zinc and iron-bearing material.

The cheinistr involved in the process may be outlined as ollows: A solution of sodium hydroxide of a particular concentration will take up, is saturated with respect tozinc oxide, but not with respect to metallic zinc. In fact, the solution, after saturation with 21110 oxide, is still capable of dissolving considerable-zinc from the metallic state.

I have found, however, that' solutions which have thus taken up zinc from the metallic state in excess of the equilibrium concentration corresponding to the solubility of zinc oxide exist in a metastable state. Such metastable solutions gradually deposit zinc oxide on standing until the zinc concentration falls to the equilibrium value. This attainment of equilibrium is extremely slow at room temperatures, requiring several months. At boiling temperature, however, the equilibrium concentration is attained in a few minutes. In view of investigations made at room temperatures and at boiling temperature, it is estimated by extrapolation that at relatively low temperatures, zero degrees to 10 0., metallic zinc will be dissolved until practically all of the free sodium hydroxide has been used up as indicated by the equation:

snao n-tzwua znosn,

n1 carrying out the invention, in accordance withlmy preferred practice, the zinc and iron-bearing material is treated with a cold solution (preferably below about 30 C.) of sodium zinc-ate containing an excess of sodium hydroxide. The addition of sodium nitrate to the solution increases the speed of the reaction. After the solution has dissolved as much zinc as it can safely take up, the resulting pregnant solution is transferred to a recovery apparatus of appropriate construction, where the solution is heated with or without the addition of water. This heating causes zinc oxide to be precipitated and free sodium hydroxide to be liberated. If water was added to the pregnant solution prior to or during heating, the solution resulting from. the precipitation step must be reconcentratcd by evaporation. After reconcentratirm, this solution is then ready for the treatment of further zinc and iron-bearing material.

l/Vhen water is used for dilution of the pregnant solution, the reconcentrated solution is found to contain less zinc than corresponds to the solubility of zinc oxide. Such a solution may be heated without cansin the precipitation of zinc oxide. A vantage may be' taken of this fact by utilizing the freshly reconcentrated hot solutionas the final solution or stage of a counter-current system. In this way, more nearly complete removal of the zinc from the zinc and ironbearing .material is effected. After the zinc has approached, but not reached, a concentration corresponding to the solubility of zinc oxide therein, the solution may be cooled down and employed as the first stage in the counter-current solution system, building up now wlth zinc to the metastable condition.

Several alternative methods of recovering the zinc from the pregnant solution may be practiced. For example, the pregnant solution maybe refrigerated to cause the separation therefrom of crystals of sodium zincateQ 'These crystals may be hydrolyzed readily to form zinc oxide and free sodium hydroxide. Again, the pregnant solution may be treated with an appropriate agent such as sodium carbonate, carbon dioxide, hydrogen sulphide or tlielike, to effect precipitation of zinc oxide or other ZlIlC compounds.

The zinc oxide product obtained by the aforementioned procedures is of a high degree ofurity containing only sli ht amounts of iron, but this zinc oxide prod uct does not possess the requisite physical properties to make it generally marketable. This zinc oxide product, however, is an intermediate product of considerable value because of its ready conversion into marketable compounds of zinc, such as zinc chlo-. ride, zinc sulfate, commercial zinc oxide, etc., thus making for diversification of outut. p The production of high rade commercial zinc chloride or zinc sul ate necessitates dizing or non-reducing atmosphere at about a red heat. Such calcination destroys organic matter arising from grease and dirt on the original zinc and iron-bearing material, and also converts any ferrous iron present to the ferric condition. The latter is essential to the precipitation of iron by zinc oxide. Sodium chlorate or other oxidizing agents will also bring about this oxidation of the ferrous iron and destroy most organic matter, but theuse of these reagents is generally objectionable on account of the impurities introduced.

The present invention is of particular ad vantage in the recovery of zinc from scrap and waste products, such as galvanized iron scrap and zinc dross. In the treatment of materials containing zinc and iron which comprises subjecting the material to the action of a solvent that dissolves zinc but not iron to form a metastable solution as the zinc concentration increases, and recovering a zinc-bearing product from the resulting solution.

2. The process of recovering zinc from materials containing zinc and iron which comprises subjecting the material to the action of an alkaline solvent containing a zincate to form a metastable solution, and recovering a zinc-bearing product from the resulting solution.

The process of recovering zinc from materials containing 'zinc and iron which comprises subjecting the material to the action of an alkaline'solvent containin sodium zincate to form a metastable so ution, and recovering a zinc-bearing product from the resulting solution.

4'. The process of recovering zinc from materials containing zinc and iron which comprises subjecting the material in the presence of an alkali metal nitrate to the action of a solution of an alkali metal zincate, and recovering a zinc-bearing product from the resulting solution.

5. The process of recovering zinc from materials containing zinc and iron which comprises subjecting the material in the presence of an alkali metal. nitrate to the action of a cold solution of an alkali metal zincate containing an excess of alkali, and treating the resulting solution for the precipitation of a zinc oxide product therefrom.

G. The process of recovering zinc from materials containing zinc and iron which comprises subjecting the material in the presence of an alkali metal nitrate to the action of a cold solution of an alkali metal zincate containing an excess of alkali, and heating the resulting solution and thereby precipitating a zinc oxide product therefrom.

lll

action of a cold solvent solution of an alkali metal zincate containing an excess of alkali, heating the resulting pregnant solution and thereby precipitating a zinc oxide product therefrom, and calcining said zinc oxide product.

9. The process of recovering zinc from materials containing zinc and iron which comprises subjecting the material to a twostage solution treatment in the first stage of which the solvent is a cold solution of an ing said hot regenerated solution as the sol vent in the second solution stage, cooling the 20 pregnant solution from the second solution stage, and utilizing said cooled solution as the solvent in the first solution stage.

10. The process of recovering zmc from materials containing zinc and iron which comprises subjecting the material in two stages to the action of solvent solutions of an alkali metal zincate containing an excess of alkali, the solvent solution in the first stage being cold and in the second stage being hot, heating the pregnant solution resulting from the first solution stage and thereby precipitating zinc oxide and regenerating a solvent solution, utilizing said regenerated solution while hot as the solvent in the second solution stage, cooling the pregnant solution from the second solution stage, and utilizing said cooled solution as the solvent in the. first solution stage.

In testimony whereof I afiix my signature.

WILLIAM GRENVILLE HORSCH. 

